1. Field of the Invention
This invention is related to the plastic resins industry and is particularly concerned with a method of drying plastic at a rapid rate so as to more rapidly prepare the plastic resin for use in a molding, particularly injection molding type, process. The invention is more particularly concerned with an improved apparatus for drying moldable resin particles.
2. Prior Art
Heretofore it has been customary to heat plastic resins which are to be used in molding processes by the use of warm air in order to remove moisture from the resins. However, such is time consuming as the plastic must be heated at a carefully chosen rate in order to prevent the plastic resin from melting and thereby becoming undesirable, and in many cases useless, for the molding process. Further, in order to obtain high quality products and prevent corrosion of the apparatus used, it is essential that the moisture within the resinous particles be removed. Accordingly, extensive drying periods wherein moderately low temperatures, i.e., below 250.degree. F are utilized have heretofore been necessary. Typically, many halogenated resins will decompose at high temperatures with the result that gases, e.g., HBr, are released which in turn severely affect the lifetime of the equipment due to their corrosive effect.
U.S. Pat. NO. 3,434,220 to Forster discloses that microwave heating can be used to assist in the removing of moisture from essentially nonpolar polymers in a two-step process using two different wave frequencies, the second being at 2450 mHz, the frequency of water. Forster further indicates that warm air may be used to prevent condensation, with microwave energy.
Today extensive use is made of molded plastic articles such as interiors for automobiles, furniture, novelty items and the like. It is economically necessary, therefore, to make these items as rapidly as possible.
U.S. Ser. No. 288,941, filed Sept. 14, 1972 now U.S. Pat. No. 3,834,038, by Robert J. Janda, discloses that the use of microwave energy alone is not the most optimum method of drying the resin particles, and said application further describes the desirability of using a partial pressure in conjunction with the microwave energy to dry the resin particles.
It is also known in the art that microwave energy may be used for drying purposes. However, the action, or behavior, of microwaves, and the reasons therefor are not entirely understood.
Industry has also developed what is commonly known as a drying hopper for various hygroscopic materials, also referred to in the art as Plenum Drying Hoppers, such as is commercially available from Whitlock Corporation. Such hoppers may be designed in practically any size although the capacity is generally in the range of 50-6,000 pounds. More advance models, such as those manufactured by Whitlock Corporation are in what is known as a "Modular" form wherein certain sections are easily removable and interchanging of components to increase or decrease capacity can be easily achieved.
In fact, the heretofore known methods and the corresponding apparatus do not totally satisfy the industry which is concerned with the drying of plastic resins at a temperature sufficiently low to prevent a melting or decomposition of the resin, in as rapid a time as possible.